The present invention relates generally to surface-acoustic-wave filters having poles, and more particularly to surface-acoustic-wave filters of the resonator type used, for example, in mobile communications equipment.
Like integrated circuits, surface-acoustic-wave filters, referred to below as SAW filters, are small and lightweight, and can easily be mass-produced. For these reasons, SAW filters are widely used as radio-frequency (RF) filters in devices such as portable telephone sets and pagers.
A portable telephone set of the code division multiple access (CDMA) type, for example, transmits in one frequency band and receives in an adjacent frequency band. The set accordingly requires an antenna duplexer with a pair of filters, one filter passing frequencies in the transmitting band and rejecting frequencies in the receiving band, while the other filter performs the opposite function. Each filter which must provide low passband insertion loss, steep roll-off between the passband and the adjacent stopband, and high attenuation across the full width of the stopband. These requirements are conventionally met by the use of a ladder filter, comprising one or more series-arm SAW resonators and one or more shunt-arm SAW resonators, with stopband attenuation produced by poles in the filter's transfer function.
As increasing performance demands are placed on mobile communications equipment, however, it is becoming difficult to meet the above requirements satisfactorily with a SAW ladder filter. One reason is that the passband width, insertion loss, stopband attenuation, and steepness of the roll-off depend on the properties of the piezoelectric substrate on which the SAW resonators are formed. Providing wide stopbands with the necessary high attenuation is particularly difficult.
These problems are difficult to solve by using the relationships between the passband and stopbands and the resonant frequencies (series resonance frequencies) and antiresonant frequencies (parallel resonance frequencies) of the SAW resonators in a SAW ladder filter, because of restrictions on the width, placement, and spacing of the passband and stopbands. It is particularly difficult to place wide stopbands with high attenuation close to the passband and still maintain a low insertion loss in the passband.